A gear pump includes a casing having a pump chamber formed in an interior thereof, a drive gear provided rotatably in the pump chamber, and a driven gear provided rotatably in the pump chamber and externally meshed to the drive gear so as to be driven to rotate thereby. A suction chamber that communicates with a suction port and a discharge chamber that communicates with a discharge port are formed in the pump chamber on either side of a meshing position between the drive gear and the driven gear (see Patent Document 1, for example). In this type of gear pump, fluid suctioned into the suction chamber is received in tooth grooves of the drive gear and the driven gear, transferred to the discharge chamber while tightly sealed between the tooth grooves and an inner peripheral wall surface of the pump chamber on which the gears slide, and then discharged from the discharge port.